Method of coating metal articles



Dec. 18, 1945. R. A. SCHAEFER METEOD OF COATING METAL ARTICLES FiledApril 23, 1942 I /Palp/z A. -5cllaefqr BY J J ATTOPNEY'F- Patented Dec.18, 1945- 2,391,039 METHOD OF COATING METAL ARTICLES Ralph A. Schaeier,East Cleveland, Ohio, assignor to The Cleveland Graphite Bronze Company,Cleveland, Ohio, a corporation of Ohio Application April 23, 1942,Serial No. 440,169

7 Claims.

The present invention relates to an improved method of coating a metalarticle with a substantial layer of silver by electrodeposition. Theprimary object of the invention is to provide a method which produces asubstantial layer of electrodeposited silver upon a metal 'base articleat increased speed over present known methods and.

at the same time to provide a dense and ductile @deposit.

Surface layers of silver have been successfully plated in the past butat a relatively slow rate of speed and in thicknesses usually of theorder of less than .001 inch. When it was attempted to plate heavylayers of silver in the order of .020 to .030 inch the time requiredmade it commercially impractical.

The present invention overcomes the difliculties encountered in the pastby providing a method by which thick silver deposits can be obtained inrelatively short periods of time. This is accomplished by means ofrelative movement between the solution and the article and also of theimproved electrolytic bath which will presently be described, togetherwith high current density.

It is a well known fundamental of electroplating that the rate ofelectrodeposition of metals is limited by the current density, the rateincreasing with an increase in current density, provided the otherfactors be so chosen that a satisfactory result will be obtained. Thus,for instance, high current density plating without relative movementbetween the article surface and the electrolytic bath leads to a loose,non-adherent and porous plate. It is believed that the reason for thisfaulty plating is due to the depletion of metal ions in the vicinity ofthe cathode which causes an accumulation of gas at the cathode. Inaddition to these effects, polarization of the anode may lower theefficiency-of the bath and slow down the process to such an extent thatthe original purpose is defeated and a defective plate may be produced.

Another factor disadvantageous to the use of high current densities isthe tendency of the plated metal to build up in various regions in adendritic pattern and thus present a rough, pebble-like surface which isvery unsatisfactory. A

further factor believed to inhibit electrodeposition of a desirableplate at high speed is'the formation of a st nant film on the cathodebecause of low'ermetal ion concentration.

Various means have been used to obtain relative movement between thearticle and the electrolytic bath in order to overcome the aboveobjections, but it is still diflicult to obtain a duotile, pore-freesurface with the conventional electrolytic bath used for silver platingwhen relatively thick coatings are desired, since conven-- tional bathsat high current density give a brittle porous silver :plate and theadhesion to a metal basewill not withstand repeated-stresses in thearticle.

One of the objects of the present invention is, therefore, an improvedmethod of coating metal articles with silver by electrodeposition'at anincreased speed through the use of high current density. Another objectof the invention isto provide a bath of a composition which will resuitin a ductile, pore-lree and adherent coating of silver to a metal base.A further object of the invention is the adjustment of the relativemovement between the article and the electrolytic bath and of thetemperature in a way that they will all contribute in the article beingplated by an adherent silver coating at a high speed,

To the accomplishment of the foregoing and related ends, said invention,then, consists of the means hereinafter fully described and particularlypointedout in the claims.

The annexed drawing and the following description set'forth in detailone approved method of carrying out the invention, such disclosedmethod, however, constituting but one of the various ways in which theprinciple of the invention may be used.

In said annexed drawing- Fig, 1 is a diagrammatic view illustrating aform of apparatus adapted to perform the process embodying theinvention;

Fig. 2 is a sectional View taken along the plane 2-4 as shown in Fig. 1and shows a typical plated article with the silver coating on theoutside;

Fig. 3 is a part of a modified apparatus; and

Fig. 4 is a sectional view similar to Fig. 2 with a coating on theinside of the article.

The apparatus illustrated in Fig. 1 is particularly applicable for theplating of the outside of cylindrical members with a-rela'tively heavycoating of silver.

This apparatus comprises a chamber 4, which part of the tank I 6 isenclosed in chamber 4, while the lower part is connected by acylindrical portion 2 of reduced cross-section to the delivery side of acentrifugal pump I which keeps the electrolyte IS in circulation asindicated by the arrows in the diagram After having passed through thetank IS, the electrolyte is returned through chamber 4 and a pipe I! topump. This circulation of the electrolyte keeps the electrolyte uniformand aids in preventing metal. depletion in the vicinity of the cathodethus preventing the formation of gas. It also aids in reducing thethickness of the stagnant cathode film and in keeping a highconcentration of metal next to he cathode.

A motor 6 serves as a power source for rotating the rack ll and therebythe cathode forming article 3. The driving connection is made by meansof gears I and 0, mounted on shafts ll and I8 respectively. The lattershaft also carries a commutator 9 contacting a brush II, which elementserves for establishing an electric connection between the cathode 3 andthe negative line of the direct current supply.

The modified form of apparatus shown in Fig. 3 illustrates the device asadapted for coating the inside of the article. In this case the anode 5,consisting of 'a silver rod, is arranged within the cathode 3 on abracket '20 attached to the side wall 2| of tank l6. All the other partsof the apparatus remain unchanged.

It is very essential in electroplating metals of any kind to provide thebase metal in as clean a condition as possible and for that reason thearticles before being placed in the assembly as shown, go through thefollowing baths which are familiar to the art:

1. solvent; degreasing 2. Alkali degreasing 3. Acid pickle.

Acid pickle has the further advantage of etching the base metal so as topromote good bonding between the electrodeposited silver and the basemetal.

The previously given steps clean the base metal sufficiently. However,when the article to be electroplated is rinsed in water after a pickle,which is the usual procedure of the art, a slight amount of oxidationtakes place at the surface and prevents adhesion in certain areas. Incases of alloyed steels, this condition is augmented or diminished bydiiferent steels.

In order to prevent oxidation I use a bath which contains a nickel salt,preferably nickel chloride, in high concentration, and acidify itslightly with hydrochloric acid. The bath temperature is preferably heldat 85 to .100 C. The work is taken from the pickle tank when wet withacid solution and then rinsed in :a dilute solution of acid, after whichit is placed in-the tank containing the nickel salt before the acidsolution has time to A dense coat of nickel is formed on the articlewhich can be rinsed freely with water because the nickel covers the basemetal and is more resistant to corrosion than the latter.

The nickel plating is followed by application of a silver strike whichin turn is followed by the deposition of the surface layer.

For applying the silver strike a bath comparatively poor in silvercyanide and rich in alkali cyanide should be used. A particularly goodbath for the silver strike consists of 2 grams per liter -'of silvercyanide and 40 grams per liter of potasvolts and at a current density of40 to 150 amperes per square foot. It is necessary. to have the currenton when the work is submerged in the bath to prevent formation of animmersion plate. This latter precaution is necessary to insure properbonding of the silver to the article being electroplated, since animmersion plate has very poor bond and therefore serves as a poor basefor a subsequent thicker layer of silver applied at high speed.

From the silver strike bath the article is transferred directly to thehigh speed plating apparatus, as shown in Fig. l or 3, while still wetwith silver strike solution and with the current on before the rack isimmersed in the electrolyte to prevent immersion plating in this stageof the plating as well. It is desirable for purposes of good bondbetween the silver and the base metal to have the solution moving andthe work rotating when contact between them is made. Since the standardplating baths were found to be 1msatisfactory for applying thick coatsof silver to metal base articles, baths are used according to theinvention having a composition which lies within the following limits:

The current density should not be too high, because with very highcurrent densities, it is difficult to obtain a smooth adherent deposit,but it has been found that with the above plating bath a current densityof -200 amperes per square foot with the circulation of the electrolytepreviously described gives satisfactory results. As bath temperatures Ifound 40 to C. to give very good results, but the process is, of course,not limited to these temperatures. The speed of rotation of the cathodeis rather critical because if rotated too fast in plating the outersurface it throws the solution away from the work by centrlfugalactionand if rotated too slowly the solution rotates with the cathode and doesnot fulfill the necessary conditions for electroplating. Under theconditions cited above a rotation speed of about 80 R. 'P. M. for a 3"diameter tube has proved satisfactory. If the tubes are rotated toorapidly, then the plating surface is interfered with since the solutionwill cling to the surface otol' be plated forming a stagnant film on theoath- A silver bath which has given particularly satisfactory coatingsconsists of:

"The potassium salts may be replaced in whole or in part by the samesodium salts. It is not known exactly what part the alkali hydroxideplays in the solution, but it is believed that it acts upon the anodes,performs a corrosive action with the aid of the electric current whichin turn prevents polarization which is usualy encountered in high speedplating. Another step which is highly desirable consists in filteringthe solution at regular intervals to keep solid material out of thebath, which if present tends to give a rough late or to cause metaltrees to grow on the plate. At the temperatures given above for theoperation of the silver plating bath the cyanide radical is continuallydecomposing giving off ammonia gas as an end product which must beremoved from the electrolyte by aeration.

Any part of the article which it is desired to keep free of plate may besuitably masked by the use ofrubber or various types a: organic coatingswhich are commercially available.

With the apparatus and solution as described, it has been found that asmooth, pore-l'ree layer of silver can be deposited in thicknesses of.010 to .020 inch per hour which is a materially greater rate than canbe accomplished with any known method now available. inch may be easilyplated by this process.

Another method which has. proven quite satisfactory for plating thickdeposits of silver upon a base metal with the improved electrolytic bathas described consists iii-holding the article as a cathode in astationary position and circulating the solution past it, thus obtainingthe desirable feature of relative movement between the article and theelectrolytic bath. This method is especially advantageous where itisdesired to plate the inside of small diameter tubes since the clearancebetween the anode and cathode is small and rotation of the cathode mightresult in shorting between the anode and cathode due to improperalignment.

Other modes of applying the principle of the invention may be employed,instead of the one explained, change being made as regards the processherein disclosed, provided the step or steps stated by any of thefollowing claims or their equivalent be employed.

I claim:

1. A method of producing a thick non-porous silver plate byelectrodeposition on a metal article which comprises pre treating thearticle by plating with a thin layer of nickel and a silver strikemounting said article 1or-rotation-- as a cathode and circulating anelectrolyte over said cathode while rotating the same, said electrolyteconsisting of per liter 40 to 50 grams silver cyanide, 35 to 40 gramspotassium cyanide, 40 to 50 grams potassium carbonate and to 14 gramspotassium hydroxide, the current density being 75 to 200 amperes persquare foot.

2. A method of producing a thick non-porous silver plate in thethicknesses of 0.010 to 0.020

inch per hour by electrodeposition on a metal article which comprisespre-treating the article by plating with a thin layer of nickel and asilver strike mounting said article for rotation as a cathode andcirculating an electrolyte over said cathode while rotating the same,saidelectrolyte consisting of per liter 40 to 50 grams silver cyanide,35 to 40 grams potassium cyanide, 40 to 50 gramspotassium carbonate and10 to 14 grams potassium hydroxid the current density bein 75 to 200amperes per square loot.

3. A method of producing a thick non-porous silver plate u to 0350 inchby elcctrcdepocition Thicknesses up to .250,

on a metal article which comprises pre-tr'eating the article by platingwith a thin layer of nickel and a silver strike mounting said articlefor rotation as a cathode and circulating an electrolyte over saidcathode while rotating the same, said electrolyte consisting of perliter 40 to grams silver cyanide, $5 to 40 grams potassium cyanide, 40to 50 grams potassium carbonate and 10 to 14 grams potassium hydroxide,the current density being 75 to 200 amperes per square foot.

4. A method of producing a thick non-porous silver plate byelectrodeposition on a metal article which comprisespre-treating thearticle by plating with a. thin layer of nickel and a silver strikemounting said article for rotation as a cathode and circulating anelectrolyte over said cathode while rotating the same at a rate of 80rotations per minute, saidelectrolyte consisting,

of per liter 40 to 50 grams of silver cyanide, 35 to 40 grams ofpotassium cyanide, 40 to 50 grams of potassium carbonate and 10 to 14grams of potassium hydroxide, the current density being 75 to 200amperes per square foot.

5. A method of producing a thick non-porous silver plate byelectrodeposition on a metal article which comprises pre-treating thearticle by plating with a thin layer of nickel and a silver strikemounting said article for rotation as a cathode and circulating anelectrolyte over said cathode while rotating the same, said electrolyteconsisting of per liter of 40.to 50 grams silver cyanide, 35 to 40 gramspotassium cyanide, 40 to 50 grams potassium carbonate .and 10 to 14grams potassium hydroxide, the temperature of the bath being held atabout 40 to C., the current density being to 200 amperes per squarefoot.

40 ing with a thin layer oi nickel and a silver strike mounting saidarticle for rotation as a cathode and circulating an electrolyte oversaid cathode while rotating the same, said electrolyte consisting of perliter 40 to 50 grams oi silver cyanide, 35 to 40 grams of potassiumcyanide, 40 to 50 grams of potassium carbonate and 10 to 14 grams ofpotassium hydroxide, the current density being 75 to 200 amperes .persquare foot.

7. A process for silver plating of a metal article which comprisespretreating the article by plating with a thin layer of nickel and asilver strike root.

RALPH A. SCHAEI'ER.

